(1) Technical Field
The invention relates to a method of enhancing the alignment between the layers of a semi-conductor product.
(2) Background Art
In semiconductor manufacture, it is generally known that alignment must be maintained between the "critical masking steps" that are carried out on respective layers of a semiconductor product. "Critical masking steps" are those process steps (e.g. etching, implantation, etc.) carried out through masks which must be aligned with respect to one another. Misalignment between these masking steps seriously degrades the reliability of the process to perform its intended function.
Previous solutions to this problem have involved the use of alignment marks referred to as "bench marks". These marks are apertures formed in the surface of the semiconductor substrate. All subsequent masking steps are aligned with respect to these bench marks.
More recently, the industry has found that the bench mark alignment system does not provide enough accuracy as device dimensions are reduced. Assume that four critical masking steps are used in a given process. Since the fourth mask is directly aligned to the bench mark, it is only indirectly aligned to the other masking steps. For example, if it were necessary to align the fourth masking step with respect to the second masking step, only a "second order alignment" would be obtained. That is, the direct (i.e. "first order") alignment between these two masking steps depends upon the accuracy of the alignment between each of two masking steps and the bench marks. It has been found that any alignment below a first order alignment (i.e. a direct alignment between the desired masking steps) greatly degrades the density of the aligned structures.
In general, first order alignment can be maintained by forming alignment marks in the layers upon which the critical masking steps are carried out. If an oxide layer is grown on the substrate, a groove or similar surface discontinuity can be formed therein to form an alignment mark indicative of the positioning of a critical masking step (e.g an etch through a mask) carried out on the oxide. The next critical masking step in the process is then aligned with respect to the alignment mark in the oxide. Similarly to conventional bench marks, these "first order alignment marks" are formed at a peripheral location on the wafer removed from the product regions, such as a kerf region.
The inventors encountered difficulties when they attempted to use first order alignment marks in automatic alignment systems. These alignment tools require high visibility contrast between the alignment marks and the layers in which they are formed. In general, the visibility contrast is a function of the light diffraction/reflection properties of the given layer(s), as well as the light transmissive properties of any surrounding layers, which in turn are related to the thickness and composition of these layer(s). Contrast can also be adjusted by controlling the depth and sidewall topography of the alignment marks which are formed in the layer.
This latter approach is known in the art for enhancing the visibility contrast of conventional bench marks. See e.g. U.S. Pat. No. 4,374,915 (issued 2/22/83 to Ahlquist et al and assigned to Intel Corp.). After the bench marks are formed in the substrate, the bottom surfaces of the marks are roughened in order to scatter incident light and thus increase visibility contrast between the marks and the substrate. The bottom surface of the marks is etched through the same photoresist mask which is used to create the marks in the substrate. Note that this process is non-selective; that is, all of the alignment marks are enhanced simultaneously and to an equal extent. A similar process is shown in an article by Helmeyer at al "E-Beam Registration Mark Enhancement By Pyrocatechol Etch", IBM Technical Disclosure Bulletin, Vol. 24, No. 9, Feb. 1982, pp. 4731-4732.
The above processes are inappropriate for first order alignment marks. For these marks, an enhancement process is needed which can selectively enhance selected ones of the marks. That is, while some marks may require little or no enhancement, others may require a great deal of enhancement. For example, a critical masking step may be carried out which does not include etching into the underlaying layer (e.g. implantation). In this case, an extra etch would have to be performed to form the first order alignment mark in this layer. The above-described processes do not provide such a selective enhancement capability.